This invention relates to an optical head for recording desired information on a medium such as a magneto-optical disk and for reproducing recorded information, and more particularly to a high-density recording optical head for detecting only light reflected from regions of the medium where information is actually recorded, in order to increase the recording density of the medium.
In processing information optically, in order to process large amounts of information in a short period of time, it is necessary to record as much information as possible on one piece of medium. Accordingly, for recently popular magneto-optical disks, for example, an iristor method in which a reproducing layer is added to a recording layer has been suggested. Nevertheless, in such a method, high production costs due to disk structure and the necessity for complicated operational pick-up structures raise difficulties in actual applications.
Another method for increasing recording density requires minimization of the unit of information, and accordingly, minimization of the diameter of a light spot formed on a disk. However, if laser light is concentrated by a convergent lens, only a narrow beam may be formed at the focus point, but it does not diminish infinitely. Therefore, there is a limit to the possible improvement of recording density according to these methods.
A more detailed description of the above with reference to FIG. 1 is as follows.
In a conventional optical head, during recording, light incident on the disk D is emitted from laser diode 1 and is collimated by collimating lens 2, is given a circular cross section by passing through beam shaping prism 3, and is concentrated onto disk D by objective lens 5. Light reflected from disk D is divided into two beams by beam splitter 7. One of these beams reaches photodiodes 10 and 10' for detecting a reproduction signal, and the other beam reaches photodiode 13 for detecting a control signal. In the figure, a reference numeral 4 is a reflecting mirror, 6 is a monitor photodiode, 8 is a half-wavelength plate, 9 is a polarization beam splitter, 11 is a condensing lens and 12 is a double edge.
According to this drawing, the magnitudes of the light spot formed on disk D during recording and the light beam detected by photodiodes 10 and 10' during reproduction are determined by objective lens 5, that is, a convergent lens. However, as mentioned above, the diameter of a light spot concentrated by objective lens 5 does not diminish infinitely, and the diameter D is given by EQU D=C.multidot..lambda./(NA)
where,
C is a proportionality coefficient, PA1 .lambda. is the wavelength of the light of the spot, and PA1 NA is the numerical aperture of the objective lens.
Thus, if C=0.7, .lambda.=830 nm, NA=0.5, a light spot has D=1.2 .mu.m. Moreover, since the diametrical light intensity of such a light spot has a gaussian distribution. In this distribution, only light with strengths above a certain curie temperature around the photo axis may be recorded as a signal on the magneto-optical disk.
Therefore, as shown in FIG. 12A, the diameter d of the recording region Sd of a signal recorded on a recording face WF between grooves G of a disk is only about 0.5 to 0.6 times the diameter D of the light spot region SD formed on the recording face WF. If the only light reflected from the recording region Sd of diameter d can be detected, a desired reproduction signal may be obtained without interference with light reflected from the light spot region SD with diameter D excluding the recording region Sd with diameter d, and in this case, the interval between recording regions Sd on recording face WF during recording may be narrowed, allowing the increase of recording density.
However, in a conventional optical head, the diameter of a light beam detected by a detection device is determined by the capability of the objective lens 5 to concentrate the light beam, and the reflected light from the entire light spot region SD with diameter D is picked up by the objective lens 5. Accordingly, the interval between recording regions Sd could not be reduced to be less than D.
Meanwhile, according to Japanese Patent publication No. Sho 61-1050122, use of a wavelength transformer halving the wavelength of light emitted from a semiconductor laser in order to increase information recording density has been disclosed. Nevertheless, light efficiency is reduced by about 90% during operation of the wavelength transformer, preventing practical application.